It seems to be a version of the Aisling, the coded political song disguised as a love song that was a standard mode in 18th-century Ireland: a young man wandering meets a beautiful mysterious woman, often by a riverside, and she mourns that those who will save her are fled or dead, but her saviour will return.
Yeats's play Cathleen Ni Houlihan could be said to be a riff on this mode.